The separation of carbon dioxide out of the exhaust gases from fossil-fueled power stations, such as for example combined cycle gas turbine (CCGT) power stations or coal-fired steam turbine power stations, requires a large amount of energy.
If use is made of a wet-chemical absorption-desorption method for the separation of carbon dioxide, this energy must be supplied in the form of thermal energy for heating the desorption process. Usually, low-pressure steam from the power station's steam circuit is used for this purpose.
Even if a power station which is in the course of being built is not yet equipped with a carbon dioxide separation device connected into it, there is nowadays already an obligation to demonstrate that it can be retrofitted (capture readiness). Accordingly, appropriate provisions are nowadays already being made so that at a later point in time a carbon dioxide separation device can be integrated into the power station without any problems. This type of provision affects, for example, the machine housing, which must be appropriately enlarged for the extraction of the low pressure steam.
In addition, there is a requirement that the steam turbine or the power station process must be appropriately configured for the extraction of low pressure steam. In the case of steam turbines with separate housings for the medium- and low-pressure stages, the extraction of low pressure steam at the overflow line is an easy possibility. On the other hand, in the case of steam turbines with a single-housing medium- and low-pressure stage, it is not possible to extract the required large quantity of steam from the turbine at a suitable pressure.
The extraction of steam from other sources within the power station process is often not economical, or is not possible in a suitable way. Thus, for example, extraction from a reheater line on the steam turbine produces an unbalanced load on the boiler. Extraction of higher grade steam for the carbon dioxide separation device must also be excluded, because this leads to unacceptable energy losses.